The present invention relates to an optical pickup device and an objective lens to be used in the optical pickup, and in particular, to an objective lens of an optical pickup device capable of conducting recording and/or reproducing of information for optical information recording media each being different by using a plurality of light sources each having a different wavelength, an optical pickup device and to a method for designing an optical element for the optical pickup device.
For example, there has been developed an optical pickup device capable of recording and reproducing information for optical information recording media each being different in terms of type such as, for example, DVD and CD, and the optical pickup device is now used for various purposes. As a light source of the optical pickup device, in this case, a semiconductor laser is used in general, and appropriate wavelength λ to be used for recording or reproducing information for DVD is about 650 nm, while, appropriate wavelength λ to be used for recording or reproducing information for CD is about 780 nm.
In this case, there is a problem when recording or reproducing information by using the same optical pickup device for optical information recording media each being different in terms of type such as DVD and CD. A thickness of a transparent base board (also called a protection base board) that protects the image recording surface of a standardized DVD is 0.6 mm, and a thickness of a transparent base board that protects the image recording surface of CD is 1.2 mm. Therefore, when the information recording surface of each of DVD and CD is irradiated by a light flux emitted from the light source through the same objective lens, there is the actual state of affairs that a distance from CD to the objective lens needs to be shorter than that from DVD to the objective lens, because of the difference of the transparent base board thickness. A distance from the objective lens to the surface of the optical information recording medium, in this case, is called a working distance.
In the past, therefore, when conducting recording and/or reproducing of information for CD after conducting recording and/or reproducing of information for DVD, for example, the objective lens set to the working distance of DVD has been moved to the position that corresponds to the working distance of CD. In the case of two different working distances existing in the optical pickup device stated above, when recording or reproducing at least one of the discs, it is necessary to force to the different position in the optical axis by impressing prescribed voltage on an actuator for focusing, which causes an increase of power consumption. For this problem, there is considered an idea of the structure to move the disc portion in the optical axis direction for the objective lens. However, this idea cannot be realized actually because a driving mechanism for high speed rotation for DVD and CD needs to be moved, though it is theoretically possible.
As a method for making working distances to agree with each other, an individual subject distance is set for using, for example, DVD and CD. Namely, by making a divergent angle or a convergent angle of a light flux (including a parallel light flux) that enters an objective lens to be different between an occasion to use DVD and an occasion to use CD, it is possible to make working distances for both occasions to agree with each other approximately. However, in the optical pickup device having such structure, the distance between the objective lens to one light source needs to be different from that from the objective lens to the other light source, and the structure of the optical system becomes complicated. The problem of this kind is especially conspicuous when an optical pickup device is structured by using a light source unit that is called the so-called 2-laser 1-package wherein two light sources each having a different wavelength are formed on one base board.